Flash memory with fast boot block access

ABSTRACT

A flash memory device and system include a boot block voltage pump for providing a word line voltage to the boot block of the flash memory. At least one additional voltage pump is provided to supply a word line voltage to the remaining memory blocks. The memory device can be operated according to a specification where data stored in the boot block can be read as valid data before data stored in other memory blocks can be validly read upon memory activation.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to memory devices and in particular the present invention relates to fast access of a boot block in a flash memory device.

BACKGROUND OF THE INVENTION

Typical flash memories comprise a memory array having a large number of memory cells arranged in blocks. Each of the memory cells is fabricated as a field-effect transistor having a control gate and a floating gate. The floating gate is capable of holding a charge, and is separated, by a layer of thin oxide, from source and drain regions contained in a substrate. Each of the memory cells can be electrically programmed (charged) by injecting electrons from the drain region through the oxide layer onto the floating gate. The charge can be removed from the floating gate by tunneling the electrons to the source through the oxide layer during an erase operation. Thus, the data in a memory cell is determined by the presence or absence of a charge on the floating gate.

To read the memory cells, a voltage is applied to the gate of the memory cells. By changing the threshold voltage of the memory cell transistor, the level of activation of the transistor can be measured using sense amplifier circuitry. Flash memories have a typical word line voltage which is coupled to the memory cell transistor of about 5 volts during a read operation. In low voltage memory devices, a charge pump may be needed to raise a supply voltage to an acceptable word line voltage. For example, a charge pump is needed to raise a 3 volt power supply to a word line voltage of 5 volts.

The array of the flash memory devices typically is divided into multiple addressable blocks of memory cells. Each block of memory cells can store data used during system operations. For example, when the flash memory is used in a processing, or computer system, the memory can be used to store system boot instructions. The process of booting a processor comprises loading the first piece of software that starts the processor. Because an operating system is essential for running programs, it is usually the first piece of software loaded during the boot process.

System operation requires fast access to the boot data during power-up to maintain acceptable performance for the processing system. Currently, the entire flash memory array is provided with a word line voltage on power-up that is sufficient to read any memory cell in the flash memory for retrieving the boot data. This can create problems with low voltage memory devices.

For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for allowing fast reading of system boot information without requiring a large voltage pump circuit.

SUMMARY OF THE INVENTION

In one embodiment, a method of operating a flash memory, the method comprises activating the flash memory, and providing valid processor boot data on an output data connection of the flash memory within a first time period following activation of the flash memory. The valid processor boot data is stored in an addressable boot block of memory cells of the flash memory. The method also comprises providing second valid data on the output data connection within a second time period following activation of the flash memory. The second time period is greater than the first time period, and the second valid data is stored in a second addressable block of memory cells of the flash memory.

In another embodiment, a flash memory device comprises an array of addressable memory cells arranged in addressable blocks. The addressable blocks comprise a boot block and at least one additional memory cell block. The memory device further comprises a first address decoder circuit coupled to the boot block, a first voltage pump circuit coupled to the first address decoder circuit for providing a word line voltage signal to the boot block, a second address decoder circuit coupled to the additional memory cell block, and a second voltage pump circuit coupled to the second address decoder circuit for providing a word line voltage signal to the additional memory cell block.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art flash memory device;

FIG. 2 is a timing diagram of the memory device of FIG. 1;

FIG. 3 is a block diagram of a system and flash memory device according to one embodiment of the present invention;

FIG. 4a and 4 b illustrate alternate memory maps of a memory array of the present invention; and

FIG. 5 is a timing diagram of the memory device of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific preferred embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the claims.

Prior to describing the present invention, a description of a prior art memory device is provided. See “1998 Flash Memory Data Book,” pages 1-1 to 1-27, available from Micron Technology, Inc., Boise, Id. for a description of a 265 k×8 boot block flash memory part No. MT28F002B-1, and incorporated herein. This device is a non-volatile, electrically block-erasable, programmable read-only memory containing 2,097,152 bits organized as 262,144 words by 8 bits. Writing or erasing the device is done with either a 5 v or a 12 v Vpp voltage, while all operations are performed with a 3.3 v or 5 v Vcc. Writing or erasing the boot block requires either applying a super-voltage to a reset/power-down (RP*) pin or driving a write protect (WP*) input to a high state in addition to executing a normal WRITE or ERASE sequence. The boot block can be used to store code implemented in low-level system recovery. As illustrated, the flash memory device includes a number of external signal inputs, including but not limited to, chip enable (CE*), write enable (WE*), write protect (WP*), reset/power-down (RP*), and output enable (OE*). The memory device also has a plurality of address input connections and data input/output connections.

The memory device is organized into five independently erasable memory blocks that allow portions of the memory to be erased without affecting the rest of the memory data. The boot block is hardware-protected against inadvertent erasure or writings. To allow for maximum power conservation, the memory device features a low current, deep power-down mode. To enter this mode, the voltage on the RP* pin is taken to a high state. In this mode, the current draw is substantially reduced for the memory device.

Power-up is defined as the time when power (Vcc) is applied to the memory device, or when the external connection RP* receives an enable signal. This connection is often used to place the memory device in the low power consumption mode to conserve battery-power, in applications which use a battery for the power supply. Flash memory devices typically operate under a specification which defines timing requirements of the memory device. For example, one timing specification of the above described flash memory device is the time required to read valid data following power-up of the memory device. This specification time is often referred to as Twrh. A typical time specified for Twrh is about 600 ns. As explained above, flash memory devices often include a voltage pump which provides an elevated voltage signal to word lines for reading data stored in the flash memory cells. Prior art flash memory devices provide the elevated voltage to all memory cell locations. That is, the memory is designed to allow data to be read from any memory array location within the specified time by coupling the elevated voltage to desire memory cells.

A specific reason why fast read operations from the flash memory data is necessary, relates to applications where a processor needs to retrieve boot information. As known to those skilled in the art, the boot data can contain code necessary to instruct the microprocessor in loading its operating system (OS), or other necessary system programs.

Referring to FIG. 1 and 2, a representative prior art flash memory device is described. As illustrated in FIG. 1, the flash memory device 100 comprises an array 102 divided into addressable blocks. Each addressable block can be referred to as a separate array, or sub-array. Each block has a corresponding x-decoder circuit 104 which provides word line voltages to selective memory cells located within the block. As explained, the word line voltage is typically greater than a supply voltage of the device. The memory device, therefore, comprises a voltage pump circuit 106 which is coupled to the x-decoder circuitry. As shown in FIG. 2, a supply voltage of 3 volts is pumped to a word line voltage of about 5.4 volts when an enable signal is coupled to a reset pin, RP*. In a typical flash memory device, data stored in any memory cell location can be read within 60-100 ns following an active chip enable (CE*) signal with a Trwh of 600 to 1000 ns, or more. As such, the elevated word line voltage, 5.4 volts, must be available when the cell location is accessed which follows when the chip enable signal is activated.

In large density memory devices, a load capacitance (x-decoder and wiring to the x-decoder) which must be driven by the voltage pump is substantially large when compared to the time allocated for providing the elevated voltage to each memory block x-decoder. For example, in one embodiment of a 64M memory device, a load capacitance is 1 nF can be expected. This is substantially higher than the expected load capacitance of 300 pF of the memory of FIG. 1.

It will be appreciated by those skilled in the art after studying the present disclosure, that a voltage pump circuit capable of driving this large load capacitance during the short time periods would not be economically feasible to fabricate on an integrated circuit. Further, as a supply voltage for the memory device is decreased and microprocessor speeds increase, the difficulties in providing fast word line voltages to all memory cell locations increases.

Referring to FIGS. 3-5, a flash memory system 200 according to one embodiment of the present invention is described. As illustrated in FIG. 3, the flash memory device 202 is coupled to a processor 204. The memory device includes a plurality of memory cell arrays 206 and corresponding x-decoders 208. The memory cell arrays can be illustrated at shown in FIGS. 4a and 4 b as a memory map of the entire memory system. FIG. 4a illustrates a memory map having a bottom boot block. That is, the bottom 16 kB of the memory array is dedicated to boot data. FIG. 4b illustrates a memory map having a top of boot block, where the top 16 kB of the memory array is dedicated to boot data.

For illustration, the embodiment with a bottom boot block includes a boot block of memory cell locations beginning at 00000H and ending at 03FFFH. Where memory address 00000H is typical the first address accessed buy a processor. The actual size of the boot block can vary, and is not limited to 16 kB. Either of these memory map locations can be used as a boot block start address location when the memory cell is coupled to a microprocessor device, in a processing system. As such, the microprocessor reads one of these locations to receive boot address data.

The flash memory device of the present invention includes at least two voltage pump circuits. The first voltage pump circuit 212 is coupled to the x-decoder of the boot block of the flash memory device. The second voltage pump circuit 210 is coupled to the remaining x-decoder circuits to provide word line voltages to the remaining memory device array locations. That is, fast boot data read times can be maintained by providing a separate voltage pump circuit for the flash memory boot location. In one embodiment, the voltage pump is not fully powered-down during the deep power-down mode.

The memory also contains I/O logic 220, a state machine 222, Command Execution Logic 230, Y-select gate circuitry 232, Input 240 and Output buffers 242, and other circuitry required to operate the memory device. This circuitry is known in the art, and is not further described herein.

In operation, when the flash memory experiences a power-up both voltage pumps are activated. Because the first voltage pump is dedicated to the boot block location, and the load capacitance for that x-decoder is relatively minimal, the word line voltage on node 250 provided by voltage pump 212 reaches its required level (Vpump) much quicker than word line voltages on node 260 for the remaining memory cell locations, see FIG. 5. As such, access to boot data stored in the flash memory device of the present invention can be accessed more quickly than boot data stored in a prior art flash memory device, even though the present memory device may be fabricated to have a larger density than the prior art flash memory device. For example, boot data stored in one embodiment of the flash memory device of the present invention can be read (Trwh boot) within about 300 ns. This time could be reduced following power-up from the power-down mode if the boot block voltage pump was not powered down. The remaining memory blocks (Trwh) could be read after the boot data for example within about 2000 ns because a smaller pump is used for the remaining memory blocks.

CONCLUSION

A flash memory device has been described which includes a boot block voltage pump for providing a word line voltage to the boot block of the memory. At least one additional voltage pump is provided to supply a word line voltage to the remaining memory blocks. The memory device can be operated according to a specification where data stored in the boot block can be read as valid data before data stored in other memory blocks can be validly read.

Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof. 

What is claimed is:
 1. A method of operating a flash memory, the method comprising: activating the flash memory; providing valid data on an output data connection of the flash memory within a first time period of less than about 300 nanoseconds following activation of the flash memory, the valid data is stored in an addressable block of memory cells coupled to a first voltage pump circuit of the flash memory; and providing second valid data on the output data connection within a second time period following activation of the flash memory, the second time period is greater than the first time period, and the second valid data is stored in a second addressable block of memory cells coupled to a second voltage pump circuit of the flash memory.
 2. The method of claim 1 wherein the valid data is processor boot data.
 3. The method of claim 1 wherein activating the flash memory comprises receiving a power supply voltage on a power supply connection of the flash memory.
 4. The method of claim 1 wherein activating the flash memory comprises receiving an enable signal on an input connection of the flash memory.
 5. The method of claim 1 wherein providing the valid data comprises: activating the first voltage pump circuit from a deep power-down state in which the first voltage pump circuit is not completely powered down; generating a first word line voltage using a the first voltage pump circuit; coupling the first word line voltage to the addressable block; receiving an address of data cells located within the addressable block; and reading the valid data from the data cells identified in the received address.
 6. The method of claim 5 wherein providing the second valid data comprises: generating a second word line voltage using a the second voltage pump circuit; coupling the second word line voltage to the second addressable block of memory cells; receiving an address of data cells located within the second addressable block of memory cells; and reading the second valid data from the data cells identified in the second addressable block of memory cells.
 7. The method of claim 1 wherein the second time period is less than about 2000 ns.
 8. A method of operating a flash memory, the method comprising: activating the flash memory including activating a first voltage pump circuit from a deep power-down state in which the first voltage pump circuit is not completely powered down; generating a first word line voltage using the first voltage pump circuit in response to the activation of the flash memory; coupling the first word line voltage to an addressable boot block of memory cells of the flash memory; providing valid processor boot data on an output data connection of the flash memory within a first time period following activation of the flash memory, the valid processor boot data is stored in the addressable boot block; generating a second word line voltage using a second voltage pump circuit in response to the activation of the flash memory; coupling the second word line voltage to a second addressable block of memory cells of the flash memory; and providing second valid data on the output data connection within a second time period following activation of the flash memory, the second time period is greater than the first time period, and the second valid data is stored in the second addressable block.
 9. The method of claim 8 wherein the first time period is less than about 300 ns.
 10. A method of operating a processing system comprising a processor coupled to a flash memory device, the method comprising: activating the flash memory device; providing a first memory cell address from the processor to the flash memory device, the first memory cell address identifying a first memory cell array location coupled to a first voltage pump circuit in the flash memory device where processor boot data is stored; in a first access to the flash memory after activating the flash memory, reading data stored in the first memory cell array location containing processor boot data; coupling the processor boot data on an output data connection of the flash memory within a first time period following activation of the flash memory; providing a second memory cell address from the processor to the flash memory device, the second memory cell address identifying a second memory cell array location coupled to a second voltage pump circuit in the flash memory device where boot data is not stored; in an access to the flash memory immediately following the first access, reading data stored in the second memory cell array location; and coupling the data stored in the second memory cell array location to the output data connection within a second time period following activation of the flash memory, the second time period being greater than the first time period.
 11. The method of claim 10 wherein reading data stored in the memory cell array location comprises: generating a first word line voltage signal using the first voltage pump circuit, which was not completely powered down prior to activating the flash memory device; and coupling the first word line voltage to the first memory cell array location.
 12. The method of claim 10 wherein reading data stored in the second memory cell array location comprises: generating a second word line voltage using a the second voltage pump circuit; and coupling the second word line voltage to the second memory cell array location.
 13. A method of operating a flash memory, the method comprising: activating the flash memory; in a first access to the flash memory after activating the flash memory, accessing a first memory cell location coupled to a first voltage pump circuit in a first access time; and accessing a second memory cell location coupled to a second voltage pump circuit immediately after accessing the first memory cell location in a second access time that is longer than the first access time.
 14. The method of claim 13 wherein activating the flash memory comprises receiving a power supply voltage on a power supply connection of the flash memory.
 15. The method of claim 13 wherein activating the flash memory comprises receiving an enable signal (/RP) on an input connection of the flash memory.
 16. The method of claim 13 wherein accessing a first memory cell location comprises: generating a first word line voltage using the first voltage pump circuit, which was not completely powered down prior to activating the flash memory; and coupling the word line voltage to the accessing a first memory cell location.
 17. The method of claim 16 wherein accessing a second memory cell location comprises: generating a second word line voltage using a the second voltage pump circuit; and coupling the second word line voltage to the second memory cell location.
 18. The method of claim 13 wherein the first time period is less than about 300 ns, and a second time period is less than about 2000 ns.
 19. A method of operating a flash memory, the method comprising: activating the flash memory; in a first access to the flash memory after activating the flash memory, providing valid data, read from data locations coupled to a first voltage pump circuit, on an output data connection of the flash memory within a first time period following activation of the flash memory; and in an access to the flash memory immediately following completion of the first access, providing second valid data, read from data locations coupled to a second voltage pump circuit, on the output data connection within a second time period immediately following completion of the first access of the flash memory, the second time period being greater than the first time period.
 20. The method of claim 19 wherein activating the flash memory comprises receiving an enable signal on an input connection of the flash memory.
 21. A method of operating a flash memory, the method comprising: activating the flash memory including a first voltage pump circuit that is not completely powered down and a second voltage pump circuit that is not completely powered down; in a first access to the flash memory after activating the flash memory, accessing a first memory cell location containing a boot block in a first access time; and in an access to the flash memory immediately following completion of accessing the boot block, accessing a second memory cell location in a second access time that is longer than the first access time.
 22. The method of claim 21, further comprising generating a first word line voltage signal using the first voltage pump circuit to access the first memory cell location.
 23. The method of claim 22, further comprising generating a second word line voltage signal using the second voltage pump circuit to access the second memory cell location.
 24. A method comprising: entering a deep power-down mode in which a first voltage pump circuit is not completely powered down; activating a flash memory cell including the first voltage pump circuit and a second voltage pump circuit; activating word lines coupled to the first voltage pump and a boot block of the flash memory; and activating word lines coupled to the second voltage pump.
 25. The method of claim 24, wherein activating word lines coupled to the second voltage pump comprises: activating word lines coupled to the second voltage pump after the word lines coupled to the first voltage pump are activated. 